Editing Water (section)

===As a scientific standard===

On 7 April 1795, the gram was defined in France to be equal to "the absolute weight of a volume of pure water equal to a cube of one hundredth of a meter, and to the temperature of the melting ice. For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely how massive one liter of water was. In spite of the fact that the decreed definition of the gram specified water at 0&nbsp;°C—a highly stable ''temperature'' point—the scientists chose to redefine the standard and to perform their measurements at the most stable ''density'' point: the temperature at which water reaches maximum density, which was measured at the time as 4&nbsp;°C

The Kelvin temperature scale of the SI system is based on the triple point of water, defined as exactly 273.16 K or 0.01 °C. The scale is a more accurate development of the Celsius temperature scale, which is defined by the boiling point (100 °C) and melting point (0 °C) of water.

Natural water consists mainly of the isotopes hydrogen-1 and oxygen-16, but there is also small quantity of heavier isotopes such as hydrogen-2 (deuterium). The amount of deuterium oxides or heavy water is very small, but it still affects the properties of water. Water from rivers and lakes tends to contain less deuterium than seawater. Therefore, a standard water called Vienna Standard Mean Ocean Water is defined as the standard water.